There are many paper containing products that can benefit from increased bulk, if other properties can be maintained at acceptable levels. One such property is a required minimum amount of Z-directional bonding (measured as ZDT, Scott Bond or others). Z-directional bonding, as well as other bonding in certain paper products such as tissue, is frequently in conflict with other desired properties, such as softness. However, other products, such as multi-layer paperboard, require minimum acceptable amounts of Z-directional bonding in the finished product to provide adequate strength.
Some paper products that used to be made as single-ply are now being made with improved characteristics, or more economically, with multiple layers. Multi-layer paper products are comprised of a high bulk central core having from one to five layers, or plies, sandwiched between two or more high modulus external plies. This structure creates a very stiff sheet through the "I-beam" principle and produces a product with improved stiffness, smoothness, strength, rigidity and coating characteristics. Maximizing the bulk of the middle layer(s) then gives the stiffest over all product. Z-directional bonding is necessary in finished multi-layer paper products to prevent inter and intra layer delamination.
Normally in order to gain adequate bonding, the product is treated such as by paper pressing, pulp refining, pulp starch addition etc., and one accepts the usually lower bulk which is obtained. Generally, as one of bulk or Z-directional bonding goes up, the other goes down. This is illustrated by FIG. 1 which shows Scott Bond versus bulk for different degrees of mechanical refining. The data points within the circle in the lower center of the FIG. 1 were the indicated pulps that were not subjected to any refiner treatment. Conventional wisdom teaches that as one mechanically works pulp, bulk goes down and Scott Bond goes up, as indicated by the arrow pointing in an ascending direction along the curve of FIG. 1.
The usual approach to solving this problem with respect to manufacture of core ply pulp would be to take the best available pulp, and conventionally refine it to achieve adequate Z-directional bonding. Another or additional approach involves the addition of expensive additives to attain adequate Z-directional bonding. Using such an approach, one would then merely accept the bulk which was obtained in arriving at some minimum acceptable Z-directional bonding.
Another approach is to specially select pulp from tree species that are inherently high in bulk. If the availability of a desired species is adequate, and segregation of the species is feasible, this is one approach-although an expensive one. It requires log and chip separation, chip pile segregation, and either swinging the cooking and bleaching operation with large intermediate and final pulp storage, or construction of separate pulping and bleaching mills.
Certain paperboard products are used for the containment of liquids. Such products are typically coated with a barrier film usually consisting of high molecular weight hydrocarbons or thermoplastics. The finished board is then imprinted, cut, and folded into a particular container shape, such as a milk carton. However the inner fold edge that is cut will not be coated with a barrier film, therefore directly exposing unprotected paperboard to wicking of liquid within the finished, filled container.
It would be desirable to find or develop improved methods for treating pulp which maximize bulk, enabling low bulk species to be used in a desired application, yet at the same time maintain appropriate Z-directional bonding. It would also be desirable to find or develop improved methods which minimize the wicking action of the exposed inner edge in multi-layer paperboard products which are used for the containment of liquid.